The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Diesel engines combust an air/fuel (A/F) mixture in cylinders that drive pistons that rotatably drive a crankshaft to produce drive torque. The combustion of the A/F mixture produces exhaust gas that includes nitrogen oxide (NOx). Therefore, diesel engine systems may include exhaust after-treatment systems that break down the NOx in the exhaust gas. For example, an exhaust after-treatment system may include a diesel oxidation catalyst (DOC), a diesel particulate filter (DPF), and/or a selective catalytic reduction (SCR) system.
In the SCR process, NOx reacts with a reductant when a dosing agent is injected by a dosing system into a flue or exhaust gas stream to be absorbed onto a SCR catalyst. For example, ammonia (NH3) may be the reductant resulting from a dosing agent that may be pure anhydrous ammonia, aqueous ammonia, or urea. In other words, the injected dosing agent (e.g. urea) may break down to form the ammonia (NH3), which reacts with the NOx. The following exemplary chemical relationships may illustrate the NOx reduction:4NO+4NH3+O2→4N2+H2O2NO2+4NH3+O2→3N2+6H2OThe SCR process may significantly reduce NOx, forming water vapor (H20) and nitrogen gas (N2).
However, SCR requires a minimum temperature to operate efficiently (i.e. a minimum effective temperature). In other words, when an SCR system is operating at a temperature below the minimum effective temperature, NOx reduction may be inefficient. Additionally, when NOx reduction is incomplete, there may be a release of unreacted ammonia (NH3) into the exhaust stream. This may be referred to as “ammonia slip,” and may have negative effects on emissions.